pdfGetLayerViewState

This function returns the suggested state for a layer when viewing the
PDF file. The return value is +1 if the layer should be on (visible),
0 if the layer should be off (not visible), or -1 if there is no
suggested state information for the layer.

The layer argument is an opaque layer handle, returned by
pdfGetLayer.

The pdfGetLayerPrintState function is similar, but gets the
suggested state for printing (rather than viewing).